Let me know.

Notice the change in pronunciation of the t in “let”, because it’s followed by /m/, which is made with both lips. Instead of using your tongue for the t, use your lips, as if you’re going to make a /p/, but keep it in and go straight to the /m/ without opening your lips.

The phrase means “Tell me”, but is more polite, as it suggests you will wait for the information. It’s neutral to informal in register.